Boiler system mechanical combustion air unit

ABSTRACT

One embodiment of a boiler system mechanical combustion air unit comprising a cylindrical main housing ( 102 ), a pressure sensing means composed of sensor cap ( 110 ) and sensor hole ( 124 ), a variable speed fan ( 104 ) and motor ( 106 ) driven by a variable speed drive ( 108 ) for moving a gaseous fluid at a variable volumetric flow rate, a temperature sensing means ( 114 ) for measuring the temperature of the gaseous fluid, and a controller ( 116 ) comprising a pressure transducer and electronic control. Other embodiments are described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional patent application Ser. No. 61/338,712, filed 2010 Feb. 23 by the present inventor.

FEDERALLY SPONSORED RESEARCH

Not Applicable

SEQUENCE LISTING OR PROGRAM

Not Applicable

BACKGROUND

1. Field of Invention

This invention, the boiler system mechanical combustion air unit, applies to combustion heating equipment, and is used to supply the correct amount of combustion air to combustion heating equipment. This invention is termed a molecular balance combustion air unit.

2. Prior Art

For purposes of discussion in the remainder of this document, unless otherwise stated, we will refer to all combustion heating equipment as a boiler. The particular type of combustion heating equipment does not affect the operation of this invention.

The combustion process for heating equipment requires both fuel and combustion air. The ultimate source of combustion air for a boiler, or a system of boilers, is the surrounding atmosphere. More directly, that ultimate source is the outside air. Boilers or boiler systems are typically placed within an enclosed space such as a boiler room, or heating plant, which is in turn located within a building. There are currently two basic methods for supplying combustion air, often referred to as makeup air, to the boilers located within an enclosed space. Firstly, there is open combustion wherein the heating unit draws the combustion air into the boiler room naturally from its outside surroundings. This requires the boiler room, and thus the building, to be open to the free flow of outside atmospheric air into the building. Louvered openings are commonly used for this purpose. This method for introducing combustion air is a source of inefficiency for combustion heating systems. It introduces excess air in addition to the required combustion air. For open combustion, there is not only a need to heat the cold outside air used for the combustion process, but also to heat the associated excess air brought into the building.

In an effort to increase energy efficiency, buildings are being more tightly sealed creating a resistance to the free flow of atmospheric air into the boiler room. This limits the usefulness of the open combustion method by restricting the natural supply of combustion air required for the heating process. Beyond tight sealing, the design of the building itself may not incorporate a means to provide for the free flow of outside air into the boiler room. A boiler plant located several floors below ground level in a central basement area of a building is an example of this situation. To compensate for this problem, a mechanical means for supplying combustion air is employed. This is the second approach to supplying combustion air to boilers. A fan is used to move combustion air from the outside atmosphere, usually through a duct system, directly into the boiler room. The ability to supply the correct amount of combustion air is the main problem with this mechanical combustion air method. The amount of combustion air required will depend on the heat demand to the boilers at any time.

The current technique for providing mechanical combustion air to a boiler or a boiler system is to use a fan to pressurize the boiler room with outside atmospheric air. As the boilers consume air from the boiler room to meet their combustion needs, the pressure in the room will tend to fall relative to the outside surroundings. For open combustion, this reduced pressure relative to the outside air is the driving force that draws the makeup air into the boiler room. The extent to which the pressure drops relative to the outside surroundings will depend on how well the boiler room is sealed, the relative temperatures of the outside and inside air (the relative air densities), and the outside atmospheric pressure. For mechanical combustion, the fan will pump makeup air into the boiler room attempting to compensate for the reduced room pressure. The problem with this approach is that pressurizing a boiler room does not directly translate to supplying the correct amount of combustion air to meet the needs of boilers or a boiler system. Again, the result is either an excess or shortage of combustion air to the boilers. This creates inefficiencies for building heating, and boiler operations. Currently, this is the only known method for providing mechanical combustion air to the boilers.

The overall problems with these methods are a combination of high installation cost, inefficiency and the overall complexity of running the system. For mechanical combustion air sourcing, these problems result from the control strategy used to implement this method. An efficient control strategy must be able to supply the correct amount of combustion air needed to meet solely the demand of the boilers, and that type of control function currently does not exist. The current method for controlling the pressurization of the boiler room requires the tight sealing of the boiler room. Tightly sealing a boiler room is expensive and difficult to achieve. Unless the boiler room is tightly sealed, two major problems result. First, excess cold air is pumped into the general building space while simultaneously supplying combustion air to the boilers. The excess cold air needs to be heated, increasing the heating costs, and thus the inefficiency inherent to the current method. The second problem comes from the technique for pressurizing the boiler room. This method requires a pressure transducer to reference some space outside the boiler room to create a differential pressure signal. Then using that differential pressure reading as the feedback signal to control the fan speed, outside air is pumped into the boiler room in an effort to control the room pressure. This approach is difficult to set up and to stabilize because of the very low differential pressures being measured, usually on the order of 0.001 Inches of Water. If the room is not tightly sealed it becomes difficult to control air flow and room pressurization, and creates an additional avenue for excess air to be brought into the building.

The control function used in current practice for pressurizing a boiler room requires a stable control signal generated from the pressure differential between the boiler room pressure and some external reference pressure. The external signal is usually the inside building pressure somewhere outside the boiler room, or the atmospheric air pressure outside the building. Directly referencing the outside air has always been a considerable problem, and is avoided if possible. Wind, natural air flows and barometric pressure variations create stability problems when referencing pressure to the outside of the building. Using the inside building space creates a more stable signal, but is still inefficient because of the excess cold air brought into the building. There is also the cost associated with the room pressurization method, which includes the cost of sealing the boiler room plus the total cost for installing and setting up the system as a whole.

This industry has an unmet need for another method for delivering combustion air that is more stable, efficient, and easier to employ and install; a method that will supply the correct amount of combustion air to meet the exact demand of the boilers.

SUMMARY

In accordance with one embodiment, a molecular balance combustion air unit comprises a main tube for housing the unit and providing a means for conducting combustion air from the outside atmosphere to boilers or a boiler room and including a means for connecting to piping used for conducting air from the outside atmosphere, a pressure and temperature sensing means for use in determining the flow rate of combustion air on a volume and mass basis, a variable speed motor driven fan within the main housing to move the correct volume of combustion air required by the boilers on an O₂ and air density basis, and a main controller for interfacing to the boilers and controlling the operation of the unit.

DRAWINGS—FIGURES

FIG. 1A shows a cut away side view of the combustion air unit with internal parts.

FIG. 1B shows a front view of the combustion air unit.

FIG. 1C shows a top view of the combustion air unit.

FIG. 2A shows a front view of the sensor cap construction.

FIG. 2B shows a top view of the sensor cap construction.

FIG. 2C shows a side view of the sensor cap construction.

FIG. 3 shows a standard fan curve illustrating the operation of the combustion air unit.

FIG. 4 illustrates a combustion air unit located in a boiler room.

DRAWINGS Reference Numerals

-   102 Combustion air unit main housing -   104 Combustion air unit fan -   106 Combustion air unit fan motor -   108 Combustion air unit fan speed controller -   110 Combustion air unit sensor cap on main housing -   112 Pressure sensor tubing -   114 Temperature sensor -   116 Combustion air unit main controller -   118 Combustion air unit motor mounting plate -   120 Combustion air unit flow straighteners -   122 Combustion air unit inlet flange -   124 Sensor hole location -   126 Combustion air unit outlet flange -   202 Sensor cap tube fitting -   204 Sensing hole location in combustion air unit main housing -   Psf Standard fan curve static pressure axis -   Q Standard fan curve volumetric flow rate axis -   P1 Measured pressure for fan speed 1 -   Psp Measured pressure at Set Point -   P2 Measured pressure for fan speed 2 -   N1 Fan speed 1 -   Nsp Fan speed at set point -   N2 Fan speed 2 -   Q1 Volumetric flow for fan speed 1 -   Qsp Set point volumetric flow -   Q2 Volumetric flow for fan speed 2 -   402 Boiler -   404 Combustion air unit

DETAILED DESCRIPTION FIGS. 1A, 1B, 1C, 2A, 2B, 2C, and 4—Preferred Embodiment

One embodiment of the combustion air unit is illustrated in FIG. 1A (side view with internal parts), FIG. 1B (front view), and FIG. 1C (top view). FIG. 4 shows an example that illustrates the relationship of the combustion air unit of this embodiment to other equipment in a typical boiler room. This particular example in FIG. 4 includes three boilers 402 in a common flue configuration incorporating mechanical venting. The combustion air unit 404 provides the correct amount of combustion air as needed by the boilers by varying the combustion air flow rate to meet that demand.

The combustion air unit comprises a tubular main housing 102 made from uniform sheet material. In this embodiment the main housing is a rolled cylinder of constant diameter along the main length of the cylinder. This sheet material can be a metal such as standard steel, galvanized steel, stainless steel, etc., the choice of which depends on the environment in which the combustion air unit will be employed. Standard steel would satisfy a typical installation. A plastic material could be employed provided it had sufficient structural strength for the application. The combustion air unit in this embodiment has a flanged inlet connection 122 for easy attachment to standard piping used to convey outside air to the combustion air unit, and a flanged outlet connection 126 which is the discharge point for the combustion air to the boiler room or boilers. The discharge end could include metal grating as a protective finger guard attached to the flange.

This embodiment of the combustion air unit includes a fan 104 attached to a variable speed fan motor 106. The fan motor is contained within the main housing in this embodiment, but could also be positioned outside the main housing. With the motor contained within the main housing, a direct drive shaft to the fan itself can be employed thus simplifying the design. The material choices for construction of the fan would follow the same reasoning as that applied to materials for the main housing. A variable speed motor drive device 108 powers the motor and receives a control signal from a main control device 116. One embodiment uses a three phase motor connected to a variable speed three phase motor drive to provide speed control of the fan. A brushless DC motor and appropriate controller is an example of another type of power means that can provide a variable speed drive to the fan. In this embodiment, the motor drive uses a standard 120 VAC power source allowing for the incorporation of a simple installation means. This embodiment incorporates a standard electrical plug allowing for a simple plug and run installation. It could also be hardwired to the electrical power source.

Pressure and temperature control signals are provided through a static pressure sensing cap 110, and a temperature sensor 114 respectively. For this embodiment, a pressure sensing port 124 consisting of an approximately ¾″ hole is placed in the main housing centered under the sensing cap. The static pressure sensing cap provides a pressure reading to a pressure transducer incorporated with the main control device via a pressure sensor tube 112 that is attached between the pressure transducer and the pressure sensing cap. The main controller receives a pressure signal from this pressure sensing means. Any available temperature sensing device can be used for the temperature sensor and is connected to the main control device sending a temperature signal to the controller. This temperature sensor measures the temperature of the air as it moves through the main housing. If a heat conductive material such as steel is used for the housing, the temperature sensor can be attached directly to the housing itself and will sense the temperature of the housing unit. This would give an equivalent temperature reading for the combustion air itself. This embodiment uses a Dallas Semiconductor DS600U+ integrated circuit chip on a circuit board as a temperature sensor. This circuit is mounted in direct contact with the metal housing. A motor mounting plate 118 provides support for the fan and motor assembly, and additionally acts as a flow, straightener. This flow straightening means is usually sufficient for most applications. Additional flow straighteners 120 can be added between the fan motor and the pressure sensing port as needed. Flow straighteners, if necessary, can help to provide a more stable static pressure reading at the pressure sensing port, and can also help to eliminate fan system effects.

FIG. 2A (front view), FIG. 2B (top view), and FIG. 2C (side view) show the sensor cap. The location of the sensing port hole 204 is shown centered under the sensing cap. The pressure sensor cap is located at the inlet side of the combustion air unit. For this embodiment, the pressure sensing cap is placed a distance from the inlet of the combustion air unit, approximately equal to the main housing diameter. The cap and its position provide a stable static pressure reading from which the combustion air unit operates. The pressure sensing cap is kept a distance from the fan of at least 2 main tube diameters. The sensing cap for this embodiment is approximately 2 inches by 2 inches in the base dimensions, and approximately 1.5 inches in height. A sensor cap tube fitting 202 is placed in the side of the pressure sensing cap centered at approximately ½ inch from the top of the cap. The pressure sensor tube 112 is attached to the sensor cap tube fitting and runs to the pressure sensing means. This pressure sensor tubing can be made of a flexible material such as rubber tubing, or a rigid material such as stainless steel metal pipe. The appropriate sensor cap tube fitting is used depending on the type of pressure sensor tubing used.

The controller 116 in this embodiment is an electronic controller which could incorporate any of a number of current methods for executing a sensing and control action for the proper functioning of the combustion air unit. These methods could be with a microcontroller, a microprocessor, an FPGA, a CPLD or an ASIC, to name a few of the most common means. The controller for the combustion air unit also includes a means, usually a pressure transducer, for converting the pressure from the sensor tube into a signal useable by the controller, and a means for connecting to the temperature signal from the temperature sensor. Included in the controller is a means for connecting to each of the boilers which will allow for the identification of the boiler and its current operating state. The controller has a means for connecting to and transmitting a speed control signal to the motor speed controller. The abilities and techniques for designing such a controller are within the current state of the art. What is new is the control strategy behind the algorithm incorporated within the controller for executing the sensing and control functions.

Operation—FIG. 3

The basis for the operation of this molecular balance combustion air unit is controlling the volumetric flow of combustion air as required by the boilers within the boiler system by adjusting the fan speed of the unit, and using the measured pressure at the pressure sensing port as a feedback control signal. Given a controller as mentioned above, it is easily within the art to readily develop an algorithm for the selected controller that executes the sensing and control functions, provided a workable control strategy is known. This section teaches that strategy.

A key point in devising the control strategy for the combustion air unit is the realization that the heat demand from a boiler system is constantly changing, resulting in a constantly changing firing state for the boilers and consequently, a constantly changing demand for combustion air. A unique aspect of the control function devised for this combustion air unit is that the control function set point is the combustion air demand, and is consequently constantly changing. This is a unique approach in that a typical control function has a fixed set point and a control strategy that adjusts the operating state of the system to match that set point. For the combustion air unit of this invention, the set point changes as the combustion air demand changes, and then the fan speed is controlled to deliver the correct amount of combustion air by using the inlet pressure and the related fan curves as the basis for measuring and controlling the combustion air flow rate.

For discussion, the control strategy behind the operation of the combustion air unit is described in three parts.

The first part is the means for determining the quantity of combustion air needed by each boiler for each of the boiler firing states. This data, entered into the controller during setup, is required in two forms: as a volumetric flow rate, and as a mass flow rate. One of the complex features inherent to the operation of this combustion air unit is that the boilers require combustion air to be delivered on a mass basis, but a fan can only control the delivery of combustion air on a volumetric basis. A unique feature of this invention is that a means has been created to implement this type of operation. Once the combustion air data, used to operate the combustion air unit, has been determined, it can be entered into the controller.

The second part is the means for establishing the control set point, which is the combustion air demand of the boilers in the boiler system, and is required in two forms as described above, a mass flow rate and a volumetric flow rate.

The third part to the strategy is the actual method used to control the delivery of combustion air based on the constantly changing set point.

For the first part, a description of a method for determining the correct amount of combustion air volume required by the boilers for each boiler firing state is provided. Then a means for entering the data into the controller is described. Finally, the means for informing the controller of the current firing state of each boiler is described. The volumetric data for this part can either come directly from the boiler manufacturer, if available, or it can be calculated. The following describes the basis of a method for calculating the combustion air requirements for a boiler from the standard boiler specifications. Standard boiler specifications are the fuel type, the boiler MBH (thousands of BTU's per hour) and the % CO₂ in the flue gas, for each of the boiler firing states. In some instances the % excess air is provided instead of the % CO₂. This data is standard information from boiler manuals, readily available from the manufacturer.

To determine the required mass and volumetric flow rate in lbm/min and CFM (ft³/min), respectively, for all the firing states of a boiler, begin with the fuel type, the heating capacity in BTU's for each boiler firing state, and the % excess air required for each boiler firing state. If % CO₂ instead of % excess air is provided, the % excess air can be calculated from the % CO₂. Using the standard density of air, the mass flow rate in lbs mass/min can be determined from the volumetric flow rate. This flow rate data would be entered into the controller during the installation and setup of the boiler system.

The basis for calculating the combustion air requirements comes from the chemistry of combustion. Take the case for the fuel type being standard natural gas with a composition of 80% methane plus 20% ethane. The following is the balanced chemical reaction for combustion of this fuel with k being the excess air fraction. For example, 30% excess air gives a k=0.3. The reaction basis is 1 cubic foot of natural gas at STP.

(1+k)9.2N₂+(1+k)2.3O₂+0.8CH₄+0.2C₂H₆→(1+k)9.2N₂ +k2.3O₂+1.2CO₂+2.2H₂O

Solving the right side of this equation provides the relation for % CO₂ versus % excess air.

% CO₂=120.0/(11.5k+12.6) or  (e1)

k=(120.0/(11.5*%CO₂))−(12.6/11.5).  (e2)

Equation e1 provides the % CO₂ given the excess air fraction. Equation e2 provides the excess air fraction k given the % CO₂ in the flue gas. From the left side of the balanced chemical reaction, the amount of combustion air required for each cubic foot of natural gas, NG, burned is (1+k)9.2N₂+(1+k)2.3O₂=(1+k)*11.5 air. The required combustion air at STP per cubic foot of fuel is

1 ft³NG=(1+k)*11.5 ft³.  (e3)

The combination of equation e3, the heat demand for the particular boiler firing state, and the standard heat content of this natural gas will give the total amount of combustion air required. The heat content for this standard natural gas at STP is 1142 BTU/ft³. The method is the same for all other fuel types. What changes are the hydrocarbon constituents and the coefficients in the equation and the heat content of the fuel, which normally leads to different results for the excess air equations, e1 and e2. The required data is readily available from standard engineering handbooks.

One more procedure will be required to complete this part of the strategy, and allow for the efficient operation of the combustion air unit. As shown by the chemical reaction above, the basis is the mass of constituents and not the volume. The fan within the unit, on the other hand, delivers on a volume basis. A means is needed to determine the mass flow rate through the combustion air unit, and convert this to the correct corresponding volumetric flow rate that the fan will be capable of delivering. This feature of this invention is a new method not currently employed at this time. The combustion air unit of this invention delivers the correct amount of combustion air on a molecular basis rather than simply pumping volumes of air into the boiler room, and is why it is called a molecular balance combustion air unit.

From the combustion air temperature, measured by the combustion air unit, the density of the combustion air can be determined. For this case, dry air can be assumed, and any error in calculating the density based on this assumption will be within acceptable limits. From the measured temperature T in ° F., the density can be determined from the following relation:

ρ=39.666804/(T+459.58).  (e4)

The boiler specifications provide the required mass flow rate of air in lbm/minute to the boiler depending on the firing state of the boiler. The next step is to sum up all the required mass flow rates for all active boilers which we give as M. From these two pieces of data, the required volumetric flow rate Q in ft³/minute that must be supplied by the combustion air unit can be determined using the relation:

Q=M/ρ.  (e5)

This Q is the set point for the combustion air unit control function from which the fan will operate. This number is derived from the mass flow rate of combustion air as required for correct boiler operation.

An example will clarify all the steps in this method.

Example

-   -   Assume a single stage boiler operating with the following         specifications: 35 BHP @ 6.5% CO₂. Also, assume from the         temperature sensor on the combustion air unit, that the         combustion air temperature measures 20° F. From the % CO₂ and         relation e2 above, calculate the % excess air. This gives         k=0.5097. Use the conversion factor that 1 BHP=33.4938 MBH to         convert 35 BHP=35*33.4938=1,172.283 MBH which is 1,172,283         Btu/Hr. Dividing this BTU requirement by the 1142 BTU/ft³         available heat content mentioned above, gives the required         boiler natural gas flow rate of 1026.518 ft³/hr. Thus, from this         amount of natural gas and the relation e3 above, the required         combustion air for the boiler         Q=1026.518*(1+0.5097)*11.5=17,822.944 ft³/hr. Divide this number         by 60 to get the calculated combustion air volume, Q=297 CFM         (ft³/min). Using the density (ρ) of air at 0.074887 lbm/ft³         (STP), gives the mass flow rate M=ρ*Q=0.074887*297=22.241         lbm/min. M is the mass flow rate set point for the controller.         Using relation e4, the density of the combustion air stream from         the unit is ρ=39.666804/(20+459.58)=0.082712 lbm/ft³. Using         relation e5, the volumetric flow rate for the combustion air         unit is Q=22.241/0.082712=268.9 CFM (ft³/min). This is the         number used by the controller for this boiler to establish the         set point for controlling the delivery of combustion air by the         combustion air unit. You will notice that simply pumping air         into the boiler room as is the current practice, delivers 297         CFM vs the actual required flow rate of 268.9 CFM. This is the         excess air that creates inefficiency. The process followed here         is the same for any fuel type. ASHRAE is an excellent source for         data about the different types of fuels.

This describes the means for determining the mass and volumetric flow rate of combustion air that must be delivered for each of the boiler firing states. The total amount of combustion air at any point in time would be the sum of the required amounts of combustion air for each individual boiler.

The second part needed to implement a control strategy is the method by which the controller senses the boiler firing states, and then establishes the set point for the control function. Individual boilers are activated to a particular firing state by the boiler call signals, which come from a sequencing controller. A sequencing controller is a standard component of a boiler control system to which all of the boilers are connected. The combustion air controller is similarly connected to these same signals. Methods for electronically interfacing these boiler call signals to the electronic control of the combustion air unit are well known in the industry. Wireless controls are ideal for this type of application. By this method, the combustion air unit always knows the firing state of the boilers and can determine the required amount of combustion air to deliver. This amount of combustion air is the set point used in the control strategy. During installation, the combustion air unit is set up through the controller 116 by entering the data for the required amount of combustion air for every firing state for every boiler. This is typically done through keypad entry. This constitutes part two, the means by which the controller senses the boiler firing states, and establishes the set point for the control strategy.

To recap the first two parts, an electronic controller is connected to the call for heat signals sent to the boiler units by the sequencing controller. A wireless controller is ideal for this type of application. Data for the correct amount of combustion air on a molecular basis needed by the boilers is calculated and entered into the electronic controls during installation by keypad entry to the main controller. As boilers are activated to a particular firing state by the boiler call signals from the sequencing controller, the combustion air controller adjusts the set point accordingly, using the actual measured temperature of the combustion air stream.

The third part is the method by which the combustion air unit delivers the correct amount of combustion air using the measured static pressure.

Combustion air flow control is carried out by controlling the pressure at the pressure sensing port 110 and 124 through the fan speed adjustments. The static pressure is a measure of the volumetric flow rate through the unit based on the standard fan curves. Via the fan curves, the measured static pressure is converted to the volumetric flow rate through the unit. The basic technique is as follows. If the volumetric flow rate, as measured through the static pressure and converted via the fan curves, is less than the set point flow rate, the fan speed is increased. If the measured flow rate is more than the set point flow rate, the fan speed is decreased. The trick is in the technique for determining the volumetric flow rate from the pressure measurement.

A standard fan curve typical of the fan system in this invention is shown in FIG. 3 and labeled N1. The AMCA standard methods for creating these fan curves are well documented in the industry. Using the AMCA standards/N1 is a typical base fan curve measured for this combustion air unit operating at speed N1. The fan curve for the combustion air unit comprises the static pressure (measured at the sensor port) versus the volumetric flow rate based on air at standard conditions and with the fan operating at a fixed speed. FIG. 3 shows three curves labeled N1, Nsp, and N2, corresponding to three different fan speeds. For this case, N2 is greater than Nsp, which is in turn greater than N1. Because this fan system satisfies the condition of dynamic similitude, all additional curves, such as Nsp and N2, can be calculated from the base curve. From the curve, with the fan operating at a set speed, N1, a measured static pressure from the pressure sensing port, P1, corresponds to a set volumetric flow rate, Q1. This is the source of the volumetric flow rates, given the static pressure measured at the sensor port. As can be seen from the curves, as the fan speed increases, the static pressures and corresponding volumetric flow rates increase, and vice versa.

The controller will need the fan curves in a mathematical form. A common method used to create a mathematical form is to build a piece wise linear curve fit of the fan data from the measured data points. This has the form P=kQ+n on data intervals such as Pa,Qa to Pb, Qb. The piece wise curve is determined from the following relations:

k=(Pa−Pb)/(Qa−Qb)  (e6)

and

n=Pa−kQa.  (e7)

An AMCA standard fan curve is derived at a fixed fan speed, and at standard air density, usually ρ=0.075 lbm/ft³. When satisfying the condition of dynamic similitude, the fan curve for all other speeds and densities can be determined from the following fan relations

Q=Q _(b)(N/N _(b))  (e8)

and

P=P _(b)(N/N _(b))²(ρ/ρ_(b)).  (e9)

In these relations, Q is the volumetric flow rate, N is the fan speed, P is the fan static pressure, and ρ is the air density. The subscript b indicates the data from the measured base fan curves. The new P versus Q curve is created from the base measured P versus Q data. In FIG. 3, the Nsp and N2 curves are derived from the base curve, N1.

In FIG. 3, Qsp exemplifies the target set point determined as described in parts 1 and 2 above. Using a regression technique, the control strategy is as follows. Given the existing fan speed, the corresponding fan curve is created mathematically using relations e6 through e9. The static pressure is measured and converted via the fan curve to the corresponding volumetric flow rate. As example, if the fan is operating at fan speed N2, and the measured pressure is P2, the volumetric flow rate would be Q2. Compared to Qsp, this is too high. The fan speed in this case would decrease using some typical gain increment. The resulting fan speed may be N1 from FIG. 3. The steps as given above would be repeated resulting in a measured pressure of P1 and the corresponding volumetric flow rate of Q1. Compared to Qsp, this is too low. The fan speed in this case would increase using some typical gain increment which would be smaller than previously. Repeating this process, a regression technique would zero in the correct fan speed that gives the correct volumetric flow rate. In this embodiment, an acceptable hysteresis around the calculated set point is used to provide a more stable response for the performance of this combustion air unit.

Advantages

From the description above, a number of advantages of some embodiments of the molecular balance combustion air unit become evident:

-   -   (a) Using this mass instead of volume basis, a more accurate         delivery of combustion air is provided, improving the combustion         process, improving boiler efficiency, and resulting in energy         savings.     -   (b) This plug and run design affords a simplified installation         and start up, reducing installation costs.

Conclusion, Ramifications, and Scope

Accordingly, the reader will see that the combustion air unit of the various embodiments solves current combustion air sourcing problems that plague this industry. Improved sourcing of combustion air improves the efficiency of boiler operations. Another result of the molecular balance combustion air unit is the potential for reduced design and installation costs.

Although the description above contains many specificities, these should not be construed as limiting the scope of the embodiment but as merely providing illustrations of some of the presently preferred embodiments.

Thus, the scope of the embodiment should be determined by the appended claims and their legal equivalents, rather than by the examples given. 

1. A boiler system mechanical combustion air unit, comprising: a.) a fluid conduit enabling the transmission of a gaseous fluid, b.) a means near the inlet of said fluid conduit for sensing the static pressure of said gaseous fluid, c.) a variable speed fan system for moving said gaseous fluid, d.) a means for sensing the temperature of said gaseous fluid, e.) a controller in communication with said static pressure sensing means, and said temperature sensing means, and in communication with said variable speed fan system, the controller comprising electronic circuitry capable of controlling said variable speed fan system.
 2. The boiler system mechanical combustion air unit of claim 1 wherein said sensing means is a hole through said conduit and covered by a rectangular cover.
 3. The boiler system mechanical combustion air unit claim 1 wherein said variable speed fan system is a 3 phase motor with a variable speed three phase drive.
 4. The boiler system mechanical combustion air unit claim 1 wherein said variable speed fan system is a DC motor with a variable speed DC motor drive. 